1. Field of the Invention
The invention relates to an atomic absorption spectrophotometer for quantitating metal elements contained in sample substance, and a low pressure discharge tube used as the light source for the atomic absorption spectrophotometer.
2. Description of the Prior Art
A hollow cathode lamp is generally used as the light source for an atomic absorption spectrophotometer, but a low pressure discharge tube is also used. The low pressure discharge tube is, as shown in FIG. 1, a U-shaped enclosed light permeable lamp tube 2, in which metal or metal salt 4 such as halide is sealed in an evaporable state together with rare gas such as argon. The argon is sealed in at a pressure of, for example, 40 mmHg. In the curved part of the lamp tube 2, a part for storing the metal or metal salt 4 is provided, and electrodes 6a, 6b are put in the lamp tube 2 from the base part of the straight portion of the lamp tube 2. A heater wire 8 for stabilizing the lighting of the lamp is wound around the lamp tube 2.
The luminescence of the low pressure discharge tube emitted in the direction orthogonal to the straight portion within the plane containing the straight portion is utilized. Accordingly, if the lamp intensity is strengthened, the light from the preceding straight portion is absorbed by the succeeding straight portion, and the output light intensity is not so strong, and the analytical sensitivity is not enhanced so much.
In the atomic absorption analysis, as the method for correcting the background, three types are known, that is, the D.sub.2 (Deuterium) method, the self-resorption method (SR method), and the Zeeman's method. In the D.sub.2 method, a hollow cathode lamp and D.sub.2 lamp are used as the light source, and in the self-resorption method, the discharge current value is changed only by the hollow cathode lamp. The Zeeman's method is roughly classified into the direct Zeeman's method and absorption beam Zeeman's method. In the direct Zeeman's method, an exclusive low pressure discharge tube is used as the light source. The direct Zeeman's method makes use of the phenomenon of wavelength separation of the light radiated from the light source placed in a magnetic field due to the Zeeman effect.
In the case of direct Zeeman's method, when the low pressure discharge tube in FIG. 1 is used, the hollow cathode lamp cannot be installed in its light source. Accordingly, only limited elements can be analyzed by the atomic absorption spectrophotometer according to the direct Zeeman's method.
In the direct Zeeman's method, moreover, since the light source must be placed in the magnetic field, only one type of lamp can be mounted.
In the atomic absorption spectrophotometer according to the direct Zeeman's method, since the light source is a low pressure discharge tube in the shape shown in FIG. 1, and the number of elements that can be analyzed is small, and the versatility is poor.
Yet, since only one lamp can be installed in the light source unit, it is impossible to operate to analyze sequentially and continuously by changing samples.
In the shape as shown in FIG. 1, still more, the emission intensity cannot be strengthened so much, and it is hard to raise the analytical sensitivity (S/N ratio).